Field of the Invention
This invention relates to adhesion promoting additives for organosiloxane compositions. More particularly this invention relates to combinations of additives that improve the adhesion of these organosiloxane compositions to a variety of inorganic and organic substrates, particularly glass, metals and plastics, at curing temperatures below 100.degree. C. The additives are particularly suitable for use with organosiloxane compositions that cure by a platinum group metal catalyzed hydrosilation reaction.
Organosiloxane compositions, particularly those that cure by a platinum-catalyzed hydrosilation reaction, and the properties of the cured materials obtained from these compositions are desirable for a variety of end use applications, particularly in the electrical and electronics areas. The cured materials are useful as protective coatings, potting materials and encapsulants for integrated circuits and other electronic devices and as coatings for printed circuit boards.
A shortcoming of organosiloxane compositions that cure by a hydrosilation reaction is their inability to adhere strongly to many substrates, particularly plastics and certain metals. One method for remedying this deficiency is by using primer compositions or adhesion promoting additives containing silanes and organosiloxanes with a plurality of silicon-bonded hydrolyzable groups and at least one organofunctional substituent that is bonded to silicon through at least one carbon atom. Primer compositions are applied to substrates to which adhesion is desired prior to application of the curable organosiloxane composition. Adhesion promoters are present in the curable organosiloxane composition.
Adhesion additives typically contain at least two functional groups.
U.S. Pat. No. 4,196,273 teaches using unsaturated alkoxysilanes, however, adhesion is not demonstrated at temperatures below 100.degree. C. and only adhesion to glass is demonstrated at 100.degree. C.
U.S. Pat. No. 4,082,726 describes organosilicon compounds containing epoxy functional dialkoxysilyl groups and at least one silicon-bonded alkenyl radical or silicon-bonded hydrogen atom. Adhesion with compositions containing these compounds is demonstrated after cure between 100.degree.-200.degree. C. U.S. Pat. No. 4,087,585 teaches physical blends of epoxy functional alkoxysilanes with silanol functional fluids containing alkenyl functionality. Cure was obtained at elevated temperatures.
Using multifunctional organosilanes containing ethylenic unsaturation, epoxy functionality, and alkoxy functionality as adhesion promoters is taught in U.S. Pat. No. 4,732,932.
U.S. Pat. No. 4,659,851, U.S. Pat. No. 4,719,262, and U.S. Pat. No. 4,906,686 teach reaction products of unsaturated alcohols with alkoxysilanes.
U.S. Pat. No. 4,786,701 describes hydrolysis reaction products of tetraalkoxysilanes, alkenyl or silicon hydride functional organopolysiloxanes, and optional (meth)acryloxy functional alkoxysilanes. This patent does not demonstrate adhesion at temperatures below 100.degree. C. or adhesion to two different types of substrates.
European Patent Publication (EPP) 449,181 teaches using organohydrogenpolysiloxanes containing at least two silicon-bonded hydrogen atoms and at least one trialkoxysilylalkyl group as the sole curing agent. Adhesion was demonstrated after cure at temperatures between 120.degree.-150.degree. C. Japanese Patent Publication 4/311,766 teaches using as adhesion promoters resins containing SiO.sub.4/2 and Si(OR).sub.3 units, where OR is alkoxy, in addition to ethylenic functionality.
U.S. Pat. No. 4,912,188 teaches adhesion promoting additives that are linear organosilicon compounds containing ethylenically unsaturated hydrocarbon radicals, epoxy functionality, and optional alkoxy functionality at the terminus of the silicon compound. Compositions containing these compounds did not cure after 24 hours at room temperature. Adhesion was demonstrated after curing at 150.degree. C.
U.S. Pat. No. 4,721,764 describes adhesion promoters that are organopolysiloxanes containing two silicon bonded hydrogen atoms and at least one silicon bonded hydrolyzable group or oxirane group.
EPP 458,355 teaches organohydrogenpolysiloxanes containing a linear sequence of at least five silicon atoms with each molecule containing at least three silicon-bonded hydrogen atoms, at least one epoxy group, and at least one trialkoxysilylalkyl group. Adhesion at temperatures below 120.degree. C. is not demonstrated, and the organohydrogenpolysiloxane is the sole curing agent, which limits formulating latitude.
EPP 469,890 teaches organopolysiloxane compositions wherein at least one of the alkenyl or organohydrogenpolysiloxanes contains at least two alkoxy groups and/or at least one epoxy group in the molecule. The cured products were gels which were cured between 80.degree.-150.degree. C. Gels are known to have self adherent properties.
To summarize, examples of hydrolyzable groups that are present in known adhesion-promoting organosilicon compounds are alkoxy, carboxy such as acetoxy, ketoximo such as methylethylketoximo, and amido. The organofunctional group is typically one that will react with the substrate to which adhesion is desired. Examples of suitable organofunctional groups include but are not limited to alkenyl such as vinyl and allyl, mercaptoalkyl and (meth)acryloxyalkyl.
Three classes of silanes frequently used as adhesion promoting additives are epoxy-functional silanes such as 3glycidoxypropyltrialkoxysilanes, alkenyl-functional silanes such as vinyltrialkoxysilanes and alkyl silicates such as tetraethyl orthosilicate. These silanes can be at least partially replaced by their hydrolysis/condensation products.